This invention relates to position determination apparatuses and, more particularly, to an apparatus for locating the position of a movable element in a data space. The apparatus also determines writing pressure, which can be used to convey information such as the desired width of hand-drawn lines. The subject matter hereof is related to subject matter in copending U.S. application Ser. No. 429,785, now U.S. Pat. No. 4,488,000, filed of even date herewith and assigned to the same assignee as the present application.
Graphical data systems are in widespread use for determining the position of a movable element in a data space. These equipments have many uses, the most common of which is for entering coordinate information into a computer or other processor. For example, digitizer systems are utilized to enter hand-drawn lines into a computer, to denote the positions of coordinates on a map that overlays a data area, to recognize hand-drawn characters, or to denote selected check-box areas on a worksheet. There are many other uses and potential uses as well, such as in conjunction with video games.
An early form of graphical digitizer employed a writing pen or pointer mechanically coupled to a set of arms for translating the movement of the pen into a sequence of useable information signals. More recently, graphical digitizers have been developed which are based on the principle of determining the propagation time of energy between a movable element and fixed coordinate references. Various types of energy and transmission media have been employed. In one system, a stylus generates periodic soundwaves, and circuitry is provided which measures the transit time of the soundwaves travelling through the air to spaced microphones. Since the speed of sound in air is essentially a known quantity, the coordinates of the stylus are readily determinable. Other types of systems use transmission through magnetostrictive wires or employ a resistive matrix.
In another type of system, a solid platen or tablet of sound-conductive material, typically of rectangular shape, is employed. A stylus includes a source of acoustic energy and a tip which couples the soundwave energy into the solid platen. Sound sensing transducers are disposed along to adjacent edges of the platen, and are coupled to circuitry which can be used to obtain the x and y coordinates of the stylus by determining the travel time of the sound energy in the platen between the stylus and the sensors. This type of digitizer has certain advantages and disadvantages. Among the advantages is that the acoustic energy travels with relatively high efficiency through the solid sheet, and interfering objects or noise in the environment will not likely cause a problem. Also, the surface of a video display can be used as the tablet. However, there is a substantial problem in efficiently coupling the acoustic energy into the tablet and also in efficiently and inexpensively sensing the acoustic energy at the edges of the tablet. Difficulties have been encountered in launching the sound energy into the solid platen such that it radiates symetrically and with high efficiency, regardless of the angle of the stylus.
It is among the objects of the present invention to provide improvements in the type of digitizer that employs a solid sheet for transmission of acoustic energy.